This invention relates generally to variable Venturi carburetion systems for supplying a fuel-air mixture to the internal combustion engine of an automotive vehicle, and more particularly to an improved system for automatically controlling the flow of fuel and air admitted into a variable Venturi structure to maintain a desired ratio thereof under varying conditions of load and speed in order to attain higher combustion efficiency, reduced emissions and significantly increased fuel economy.
The function of a carburetor is to produce the fuel-air mixture needed for the operation of an internal combustion engine. In the carburetor, the fuel is introduced in the form of tiny droplets in a stream of air, the droplets being vaporized as a result of heat absorption in a reduced pressure zone on the way to the combustion chamber whereby the mixture is rendered inflammable.
In a conventional carburetor, air flows into the carburetor through a Venturi tube which is generally circular in shape. The reduction in pressure at the Venturi throat causes fuel to flow from a float chamber in which the fuel is stored through a fuel jet into the air stream, the fuel being atomized because of the difference between air and fuel velocities. This "carburetion" effect is, however, operative only within a narrow range of engine speeds as predetermined by design selections which at best represent a compromise of all operating conditions in a motor vehicle.
The behavior of an internal combustion engine in terms of operating efficiency, fuel economy and emission of pollutants is directly affected by the fuel-air ratio of the combustible charge. Under ideal circumstances, the engine should at all times burn 14.7 parts of air to one part of fuel to satisfy the stoichiometric air-to-fuel ratio. But in actual operation, this ratio varies as a function of operating speed and is affected by changes in load and temperature.
To obtain maximum economy, the fuel-to-air ratio in the mixture should be maintained within close tolerances at the prescribed optimum air-fuel ratio for each mode of operation, such as "idle" while standing still, "slow-speeds" up to about 20 miles an hour, "cruising speeds" and "high speeds." The conventional practice is to provide an accelerating pump system to furnish an extra charge of fuel for accelerations, a choke system to enrich the mixture for starting a cold engine and a throttle by-pass jet for idle and slow speed, as well as a power jet or auxiliary barrels for high speed or high power operation, all in addition to the main jet.
Another reason why the maintenance of predetermined fuel-air ratios within prescribed limits is important is that the emission of pollutants as well as the power-producing efficiency are in large measure governed thereby. Thus, when the mixture is relatively low in air, carbon monoxide is produced, and when the ratio is excessively rich in fuel, unburned hydrocarbons are emitted in the exhaust. In modern engine design, the air-fuel ratio is in some instances controlled to maintain a prescribed ratio, or the control system is preprogrammed to accommodate the ratio to specific ranges of speed and load, so that the ratio, for example, is richer at slow speeds and leaner at higher speeds.
A major problem encountered in carburetion is to secure the correct amount of suction around the main jet at slow engine speeds and yet allow enough air to enter at high engine speeds to maintain the desired ratio of air and fuel. Vanturi size must, of necessity, represent a compromise for both high and low speed operation. Because the maximum power an engine can develop is limited by the amount of air it can breathe in, the Venturi size and shape should offer minimum resistance to the larger volume of air flowing at high engine speed. On the other hand, a small Venturi is desirable at low engine speeds to afford sufficient air velocity for controllable fuel metering and good fuel atomization.
The modern approach to this problem is the use of two or more Venturis arranged in series and/or two or more barrels in parallel. The multiple Venturi design serves two purposes: First, the added Venturis build up air velocity in the smaller primary Venturi, thereby augmenting the force available at the main nozzle for drawing and atomizing fuel. Second, air by-passing the primary Venturi forms an air cushion around the rich mixture discharged by the Venturi, tending to improve mixture distribution by preventing fuel from engaging the carburetor walls. Idle or very slow speed is invariably served by an auxiliary jet around the edge of the throttle plate.
However, the typical modern carburetor requires a series of additional jets and pumping systems that cut in and out as the carburetor velocity increases and decreases above and below average speed; and as the engine operation passes through successive operating modes of acceleration, cruising, high speed and deceleration. Idle or very slow speed operations both rely on an idle jet arrangement at the closed position of the butterfly throttle valve. The actions of these primary devices give rise to large fluctuations in the air-fuel ratio and thereby adversely affect fuel economy and emissions.
But fuel economy is not the only reason for maintaining steady air-to-fuel ratios; for, as pointed out in Business Week (June 21, 1976), though a new catalytic converter is available which is adapted to limit the emission of hydrocarbons, carbon monoxide and nitrogen oxides, "A steady ratio (air-to-fuel) is crucial to the new converter because it must simultaneously harbor conflicting chemical reactions." As noted in this article, "in actual operation, the ratio fluctuates with acceleration and deceleration."
Although fuel-air mixtures may be introduced to the combustion chambers of an engine by means other than carburetors, as by fuel injection, supercharging and other expedients, none of these is comparable in effectiveness with the Venturi principle for efficient atomization of volatile fuels.
Attempts have heretofore been made to provide variable-Venturi carburetors to tailor the air-fuel supply to changing engine conditions. Thus U.S. Pat. No. 2,066,544; 3,659,572 and 3,778,041 show various embodiments of a variable-Venturi carburetor.
In my above-identified copending application Ser. No. 962,883, filed Nov. 22, 1978, whose entire disclosure is incorporated herein by reference, there is disclosed an automatic system which includes a variable-Venturi carburetor for intermingling air and fuel and for feeding the air-fuel mixture in an appropriate ratio into the throttle inlet of the manifold. Air is fed into the output of the Venturi, the air passing through the throat thereof whose effective area is adjusted by a mechanism operated by a servo motor. Fuel is fed into the input of the Venturi from a fuel reservoir through a main path having a fixed orifice and an auxiliary path formed by a metering valve operated by an auxiliary fuel-control motor. The differential air pressure developed between the inlet of the Venturi and the throat thereof is sensed to produce an air-velocity command signal which is applied to a controller adapted to compare this signal with the set point of the servo motor to produce an output for governing the servo motor to cause it to seek a null point, thereby defining a closed process control loop.
The intake manifold vacuum which varies in degree as a function of load and speed conditions is sensed to govern the auxiliary fuel-control motor accordingly and is, at the same time, converted into an auxiliary signal which is applied to the controller in the closed loop to modulate the command signal in a manner maintaining an optimum air-fuel ratio under the varying conditions of load and speed.
Thus the flow of air through the Venturi structure is controlled as a function of throat air velocity by a closed process control loop whose air velocity command signal is modulated by an auxiliary signal reflecting the degree of intake manifold vacuum developed under the prevailing conditions of speed and load. In this way, the flow of air and fuel in the carburetor are correlated to cope with the transitions through the modes of operation smoothly and without hesitation within prescribed desirable ratios.
The present invention deals with improvements in the system disclosed in my copending application. While this system seeks to take into account all conditions of load and speed actually experienced in a running internal combustion engine in order to maintain a fuel-to-air ratio that is optimized for the prevailing condition, the system has certain practical drawbacks which are overcome by the present invention.
These drawbacks are not necessarily unique to the system disclosed in my copending application. For example, in a conventional carburetor as well as in the variable Venturi structure of the type disclosed in my copending application, liquid fuel drawn from a reservoir is fed in by induction, the liquid fuel intermingling with and being atomized by the incoming combustion air stream. The atomized fuel is then vaporized to facilitate combustion. In order to avoid the emission of unburned hydrocarbon products and attain high combustion efficiency, the incoming fuel should be fully atomized and vaporized in the carburetor or Venturi structure. But because of the brief atomization period, atomization of the fuel is not fully effected and optimum combustion efficiency is not realized.
The following U.S. references are pertinent to the present invention: Pelizzoni, U.S. Pat. No. 3,659,572; Kincade, U.S. Pat. No. 3,778,041; Konomi et al., U.S. Pat. No. 3,960,118; Eckert, U.S. Pat. No. 4,084,562; Hattori et al., U.S. Pat. No. 4,052,968; Eversole et al., U.S. Pat. No. 3,778,038; Abbey U.S. Pat. No. 4,118,444; Priegel, U.S. Pat. No. 3,817,227; Lawrence, U.S. Pat. No. 4,111,169; and Wahlmark, U.S. Pat. No. 1,983,225. Also of interest is the German Pat. No. 2,014,140 to Marolla.